APPENDIX B

OBSTETRIC DATES AND INTERVALS

MONTH PRENATAL CARE BEGAN

In 1988, the Vermont Department of Health adopted the National Center for Health Statistics method of calculating the month prenatal care began. This allows Vermont data to be more easily compared to national data, and eliminates any error that could be introduced by hospitals using different reporting methods.

The calculation is based on the month of last menses and the month of the first prenatal visit. For example, if the date of last menses was January 1 and the date of first prenatal visit was March 10, the calculation shows that prenatal care began in the second month.

WEEKS OF GESTATION

Live Births and Fetal Deaths: National Center for Health Statistics methodology is used to determine gestational age. The weeks of gestation are computed from the date of last menses and the date of birth, if the data are available and the computed weeks are between 15 and 55. For those records where month and year are given, but day is missing, the weeks of gestation is assigned the gestational weeks of the preceding record that has complete data with the same computed month of gestation and the same 500-gram birth weight interval.

An edit is then run that checks whether the birth weight of the record is within two standard deviations of the average birth weight for the calculated week of gestation. If the calculated week of gestation is within these bounds, it becomes the assigned week of gestation. If the calculated week of gestation is not within these bounds, but the given week of gestation is, the given week is assigned as the week of gestation. If neither the calculated nor the given week is within these bounds, the record is queried. In 1996, 91.5 percent used the calculated weeks, 7.7 percent used the given weeks, 3 records were queried to find the weeks, and the remaining records were coded to unknown weeks gestation.

Abortions: Weeks gestation is calculated from the date of last normal menses and date of abortion. The clinically estimated weeks is used if the date of last normal menses is unknown, or if the calculated weeks and the clinically estimated weeks are more than two weeks apart.

NUMBER OF PRENATAL VISITS

Since 1985, hospitals report the date of the first and last recorded prenatal visits and the number of visits recorded. In 1986, this was amended for births of 36 or more weeks of gestation with less than 20 reported visits, and whose last reported visit was within four weeks of delivery. In these cases hospitals report the number of prenatal visits listed in a patient's chart plus one visit for every week between the date of the last recorded visit and the delivery date.